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KOBA-step® Gage Bar

KOBA-step gage bar Individual gage blocks arranged obliquely in three dimensions on the table of the cmm

Step Gages for Checking the Accuracy of Coordinate Measuring Machines

In industrial metrology, actual physical bodies of known length which can be contacted by mechanical sensors have an important part to play as reference standards when measuring geometrical parameters. They have become increasingly important for assessing the accuracy of two and three-axis coordinate measuring machines (CMMs) which employ mechanical sensors. Checking the length measurements uncertainty has proved to be a highly informative and economical method for the acceptance testing and ongoing monitoring of CMMs. The Step-gage can be used for this in a great variety of ways given, for example, the advantages of uni-directional and bi-directional targeting and of measurements from all the gage faces along the line of measurement in succession while needing only a short time for preparation and measurement. Local errors can be detected in the CMM and characteristics can be derived for individual coordinate axes of the machine.

With the aid of the length measurement uncertainty, the manufacturer or user can specify and check the accuracy of a coordinate measuring machine to establish its suitability for length measurement. This fundamental task in metrology is of particular importance due to the fact that in practice the majority of measuring requirements are for the measurement of lengths. "Length measurement uncertainty" is defined by VDI/VDE guideline 2617, part 2.1 as the uncertainty with which a CMM allows the precisely known distance between two points on two mutually parallel gage faces situated in succession along a line of measurement to be remeasured. Figure 1 sows a measurement of this kind being made, taking as an example an individual parallel gage block with an outside length Le which is arranged obliquely in three dimensions and whose length is remeasured by successive contacts with the block with the probe head in positions I and II.

On the step gage, spacing of different kinds for making test measurement are all available simultaneously, as follows:

The illustrations show only one of many options available for each type and size of spacing. The value of the length measurement uncertainty is generally given in the form of a length-dependant formula: U = A + K x L </= B

Figure 2, KOBA-step Le measurement
Figure 2, Castellated step gage arranged obliquely in three dimensions on the table of a coordinate measuring machine, with an outside length Le being measured.
Figure 3, Measurement of inside and outside dimensions and position of gage face
Figure 3, Step gage arranged obliquely in three dimensions on the table of a CMM, showing measurement of an inside dimension Li, of and outside dimension Ls or of the position Lp of a gage face as a distance from the datum face.

A distinction should be made between the figure U1 specified for one-dimensional test measurements along a coordinate axis (with terms A1, K1, B1), the figure U2 for two-dimensional test measurements made diagonally in a coordinate place (with terms A2, K2, B2) and the figure U3 for three-dimensional test measurements made diagonally in the three-dimensional space defined by the coordinates (with terms A3, K3, B3).

Read more about Step-gages...

Applicable SIC Codes: 3423, 3545, 5049, 5084
Applicable NAICS Codes: 332212, 333515, 423490, 423830